1. Field of the Invention
The present invention relates to cargo handling systems and methods and more particularly to a self-stabilizing multiple sling system and method adapted for handling a plurality of stacks of discrete cargo items.
2. Description of the Prior Art
The present sling system and method is adapted for handling discrete, substantially identical cargo items arranged in adjacent stacks. It is particularly adapted to handle bales of cotton.
Heretofore the loading of cotton bales aboard a ship has been a slow and tedious operation. The bales each typically weigh 500 pounds or more. The most common device used for making up and handling drafts of cotton bales to be loaded or discharged was the rope choker sling. Such a sling is simply a length of rope or wire whose ends are formed into loops. The sling is disposed about a cotton bale, with the longer portion or rove being passed through the loop of the shorter portion or bite. The ship's cargo hook is hooked onto the rove and when the draft is lifted the sling is made taut and grips tightly onto the bale. Usually one bale at a time is handled by a choker sling, although as many as four bales have been handled in this way, depending upon the size of the bales and the working conditions.
The foregoing choker sling has proved to be unsatisfactory because of the limited number of bales that can be handled at one time. Too many bales in a choker sling draft cause instability, and there is a great danger that the draft will tip and spill one or more bales out of the draft. Moreover, depending upon the weight of the draft and the type of sling material used, the sling can cut into and damage the bales and also make it difficult to disengage the sling from the bales.
Because of the instability resulting from stacking too many bales into one draft, another handling system was advanced in an attempt to speed up the loading and discharge operations. This system utilized a large platform hoisted by wire rope bridles held apart by spreaders. The stacked bales were free standing and the number of bales handled in one hoist was thus limited by safety considerations and the size of the platform used.
Another prior art method for handling cotton bales is the well-known whip or single-fall and skid system. This employs one winch and one boom with an associated fall, together with an inclined skid leading from the pier to the deck of the ship. The cargo hook on the end of the fall is hooked onto the draft and the winch is operated to drag the draft up the inclined skid, over the bullwarks, and then over to the hatch. Other systems, such as the double-whip or split-fall system or the burton system have also been used for handling single bales of cotton.
Typically, the foregoing systems have not been able to handle more than perhaps 55 to 60 bales per hour because of the considerable apprehension about stacking more than three or four bales of cotton at a time in a sling. The bales are relatively heavy and any appreciable relative movement between individual bales can result in the load spilling.